The transformation of bacteria with vectors containing DNA sequences encoding mammalian proteins is common. Expression of the mammalian proteins by bacteria can result in high production of the protein contained within inclusion bodies in the bacteria. To obtain and purify the protein, the protein-containing inclusion bodies must be extracted from the bacteria.
In the past inclusion bodies have been isolated by centrifuging the fermentation broth, removing the supernatant liquid, resuspending the bacterial pellet in buffer and disrupting the bacterial cells by mechanical techniques such as homogenization or sonication or, alternatively, by the use of lysozyme plus detergents. The mixture containing the disrupted cells is then centrifuged resulting in supernatant liquid and pelleted inclusion bodies. An inclusion body pellet is a pellet containing inclusion bodies obtained after bacterial cell disruption and subsequent centrifugation. However, this procedure involves centrifuging fermentation broth containing live bacteria as an initial step. Often, during large scale centrifugation the live bacteria can then be emitted into the surrounding atmosphere in the aerosol emitted by the centrifuge.
U.S. Pat. No. 4,958,007 solves this problem by first inactivating cells by adding either toluene or acid to the fermentation broth. The broth is then centrifuged, the supernatant liquid removed, and the bacterial pellet resuspended in a buffer and the pH of the suspension is adjusted such that the final pH is between 6-9. Alternatively, the pH of the acidified fermentation broth is adjusted to 6-9. The bacterial cells are then disrupted by homogenization releasing the inclusion bodies from the cells. This process eliminates the emission of live bacteria into the atmosphere during centrifugation; however, when toluene is used to inactivate the bacteria, the process must be carried out in an explosion-proof room as toluene is flammable and its fumes explosive. Furthermore, when acid is used to inactivate the bacteria and the bacterial pellet is resuspended in a buffer and the pH of the suspension is adjusted such that the final pH is between 6-9, previously soluble bacterial proteins are insolubilized resulting in an inclusion body pellet containing an unacceptably high level of bacterial host proteins.
Thus, there is a need for an improved process for extracting insoluble, recombinant proteins in which all or most of the bacteria are killed prior to centrifugation, most of the host bacterial proteins remain in solution, and the predominant insoluble protein is the bacterially expressed heterologous protein.